The present invention is related to the field of pressure sensor assemblies, and more particularly to such assemblies in which the pressure sensing element or sensor within such assemblies is protected from hazardous chemicals or contaminants which may be contained in the medium the pressure of which is to be sensed.
Capacitive and piezoresistive pressure transducers are known in which, in response to the sensed pressure of a medium, an integral diaphragm is displaced. This causes changes in electrical characteristics of a component associated with the diaphragm, and these characteristics are representative of the sensed pressure. When such pressure sensors are utilized in the automotive environment for sensing engine manifold pressure, the medium, the pressure of which is being sensed, may contain hazardous chemicals such as gasoline or oil which may attack the pressure sensor and eventually degrade its performance. In addition, other environmental contaminants may be present in the manifold pressure medium such as moisture or salt, and these also can attack and/or degrade the pressure sensor and create erroneous pressure measurements.
Typically, the pressure sensor is mounted on a substrate and is contained within a mechanically protective housing mounted on the substrate. The housing and substrate essentially surround the sensor. While this provides mechanical protection for the sensor, protection from hazardous chemicals and contaminants in the medium, the pressure of which is to be sensed, must also be provided. In one typical prior pressure sensor assembly, a silicone gel or fluorosilicone gel is applied over the external surface of a pressure sensor and essentially partially fills the housing in which the pressure sensor is mounted. However, these gels, which form the direct interface between the medium being sensed and the pressure sensing element, are only somewhat resistant to contaminants in the medium being sensed. Thus, long-term exposure to harmful contaminants in the medium can still degrade the performance of the pressure sensor.
Some prior pressure sensors have contemplated filling an internal cavity within the pressure sensing element or sensor with an incompressible liquid such as silicon oil. This technique is typically used when two different liquid pressures are to be sensed and problems are encountered with respect to having one liquid fill a small volume internal cavity within the pressure sensing element between the sensor diaphragm and a base to which the diaphragm is mounted. Such oil-filled sensors essentially fill the internal cavity of the sensor with oil as a pressure interface medium. However, these sensors typically do not protect external portions of the sensing element, including the external surface of the diaphragm, from contaminants in the sensed medium external and adjacent to the diaphragm.